Filtered cathodic arc source

ABSTRACT

The plasma beam from a cathodic arc is filtered by baffles, a double bend toroidal plasma duct and a biased, corrugated liner within the duct, giving coatings virtually free of contaminating macroparticles. Reactive gas is ionised in the plasma ball and automatic arc ignition is provided.

BACKGROUND OF THE INVENTION

This invention relates to a filtered cathodic arc source. Morespecifically, this invention relates to an improved filtered cathodicarc for generating a plasma beam containing positive ions for depositinga coating of the positive ions on a substrate. The invention relates toimproved filtering of the plasma beam, to a method and apparatus forcoating substrates using a filtered cathodic arc, to a method andapparatus for generating multi-layer coatings using a filtered cathodicarc, to ignition of the filtered cathodic arc, and to substrates havinga coating of positive ions obtained using a filtered cathodic arc.

Various methods and apparatus are known in the art for obtaining a thinfilm or thin coating on a substrate. It is known to deposit films byphysical vapour deposition techniques and chemical vapour depositiontechniques; this invention relates to physical techniques. It is knownto provide such coatings using magnetron sputtering, and one such methodis described in U.S. Pat. No. 5,225,057. The quality of these films isopen to considerable improvement. Sputtering is sometimes ion beamassisted. The purpose of the ion beam may be to clean substrate prior tocoating or to promote reaction of subsequent deposited layers.

Another method of depositing thin films involves the use of positiveions generated from a cathodic arc source. The cathodic arc is a form ofelectrical discharge in vacuum which is sustained in metal plasmacreated by the arc alone and does not require the addition of an inertgas. Currents used in cathodic arc systems are typically of the order of100 amps, at around 30 volts. A large percentage of the metal vapourgenerated by the arc is ionised by the discharge and a fraction of thearc current escapes as a beam of positive ions; this fraction is steeredand optionally filtered to produce a coating on a distal substrate. Theincreased energy of these positive ions compared to the particles inprevious deposition methods is thought to be a reason why arcevaporation techniques would deposit high density, high uniformityfilms. Deposition of thin films by filtered arc evaporation is describedgenerally by P. J. Martin in Surface and Coatings Technology, Volumes54/55 (1992) pages 136-142, and further reviewed in Surface Engineering,Volume 9 (1993), no. 1, pages 51-57.

Cathodic vacuum arc systems have thus been recognized as potentially acost-effective method to produce coatings in a vacuum. However, theapproaches taken do not address the requirements needed for theindustrial applications of this technique. Such industrial system mustbe automatic, easily maintained and produce a large coating area, freeof blemishes.

It has been observed in the deposition of films using cathodic arctechnology that the plasma beam of positive ions and electrons producedby the arc is frequently contaminated by large, typically neutral,particles that are multi-atom clusters. These contaminating particlesare commonly referred to as macroparticles and can be defined asparticles visible under the optical microscope in a film deposited usingcathodic arc methods. The presence of macroparticles in deposited filmshas precluded the use of cathodic arc techniques for obtaining opticaland electronic coatings.

Much work in the art has been directed towards filtering macroparticlesfrom the plasma beam, thereby eliminating the undesirable side effectsof the presence of macroparticles in the deposited coating. GB-A-2117610uses a baffle placed directly between the cathode and the substrate toprevent macroparticles reaching the substrate. The positive ions in theplasma beam are focused around the baffle. This has the disadvantagethat there is a very low transmission of the plasma beam to thesubstrate. Further, some macroparticles reach the substrate by bouncingoff the sides of the apparatus.

U.S. Pat. No. 5,279,723, in the name of Falabella et al, describes acathodic ion source in which the plasma beam is filtered in an attemptto eliminate macroparticles by providing a bent magnetic field to guideions in the plasma beam around a 45° bend, there being no line-of-sightfrom the arc spot to the substrate. U.S. Pat. No. 5,279,723 alsodescribes using permanently fixed baffles in the plasma duct to trapmacroparticles. These confer the disadvantage that they quickly becomecovered with a dense coating of macroparticles which can fall back intothe plasma beam and therefore lead to contamination of the substratecoating. Cleaning of these baffles is awkward as it must be carried outon a partially disassembled cathodic arc source. A similar shaped ductis described in U.S. Pat. No. 5,433,836.

Neither apparatus entirely prevents macroparticles from the plasma beamarriving at the substrate; it is indicated that tests of U.S. Pat. No.5,279,723 show a figure of less than 0.2 macroparticles/cm² /minute ofcoating time for this apparatus. This latter figure, however, is notaccompanied by any precise deposition conditions thus preventing anyconfident assessment of the filtering efficiency of the apparatusdescribed.

Other known apparatus use a 90° bend duct with an axial magnetic fieldto filter out the macroparticles. Although these may achieve acceptableresults, they do not provide a long term solution for industrialequipment designed for near-continuous working. The generous amount ofmaterials coated onto the duct wall build up over time and may lead toparticles later being resputtered back into the plasma flux. Thus, onlyshort periods of use of the equipment is possible. Also, deposited filmsstill contain significant macroparticle contamination.

A third U.S. patent, U.S. Pat. No. 5,401,543, describes using aparticular cathode target material, but still suffers from high levelsof macroparticles in deposited films.

The number of macroparticles in the plasma beam can be reduced byincreasing the pressure in the vacuum chamber. However, an increase inpressure in the vacuum chamber is likely to lead to a deterioration inthe properties of the layer deposited on the substrate. When depositingtetrahedral amorphous carbon, a trace amount of H₂ can reducemacroparticles and increase transparency in the visible light range butwill compromise the density and hardness of the deposited layer, by gasinclusion in the deposited film.

It is therefore a problem in the art to generate a filtered plasma beamfrom a cathodic arc source, which filtered beam is substantially free ofmacroparticles.

Another related problem concerns a tool that is needed to ignite thearc. Once ignited, the tool is no longer required until arc re-ignition.Initiation of the arc has been typically determined visually, whereby agraphite rod fixed to a grounded stainless steel plunger is manuallymanoeuvred against the cathode and in which the movement is observed viaan observation window in the wall of the vacuum chamber. Thisarrangement is particularly awkward, especially as the window is rapidlycoated and becomes opaque. Alternatively, a fixed tungsten or graphitetrigger mounted close to the cathode surface is used to ignite the arcby the application of a high voltage. Due to this proximity to thecathode, the elements of the igniter tend to contaminate the plasmabeam.

Another problem in the art is that no satisfactory method of obtainingmulti-layer coatings is described. At present, to obtain a multi-layercoating on a substrate requires either two separate coating machines,one for producing each individual layer of the multi-layer coating, orthe depositing of a first layer using one coating machine and then thedismantling of this machine so as to replace the cathode and thenreassembly of the machine to deposit the second layer of the multi-layercoating.

Where the coating is a composite of metal and gas atoms, such asaluminium oxide or silicon dioxide coatings, this is achieved in the artby creating a plasma beam of metal ions, depositing these on thesubstrate and reacting the deposited ions with the gaseous component,such as oxygen, of the coating. These prior art techniques are ratherinefficient in that they use large volumes of gas. An improved method ofproducing coatings of this nature is required.

SUMMARY OF THE INVENTION

It is an object of aspects of this invention to provide an improvedapparatus for and method of depositing coatings of positive ions on asubstrate, the positive ions obtained via arc evaporation of a cathodetarget. Another object is to improve the efficiency of filtering aplasma beam from a cathodic arc so as substantially to remove allmacroparticles from the filtered plasma beam. A further object is toprovide a filtered cathodic arc source that can be operated efficientlyat a high vacuum and substantially without contamination of thedeposited coating by macroparticles. A still further object is toprovide improved multi-layer coating of substrates using a filteredcathodic arc. Yet another object is to provide for coating of dielectricsubstrates, such as dielectric optical substrates, using a filteredcathodic arc. A still further object is to provide improvements in arcignition of a filtered cathodic arc. Another object is to reduce gas usewhen depositing a coating having a metal and a gaseous component. Yetanother object is to overcome or at least ameliorate problems identifiedin prior art methods and apparatus.

A first aspect of the invention relates to improved filtering ofmacroparticles from a plasma beam of positive ions.

According to a first aspect of the invention there is provided a methodfor filtering a plasma beam including positive ions, the methodcomprising generating a plasma beam including positive ions from acathode target, and passing the beam through a plasma duct towards asubstrate, the plasma duct having a double bend. Typically, the duct hasa first bend with an angle of no less than 20° in a first plane and asecond bend with an angle of no less than 20° in a second plane.

The first and second planes are optionally co-incident. Though it ispreferred that the first and second planes are not co-incident but areat an angle to each other, such as at least 15 degrees, more preferablyat an angle of between 30-90° to each other.

It is preferred that the duct is toroidal. A specific embodiment of theinvention described below has a toroidal duct, and from a practicalpoint of view vacuum tubing for construction of a duct of the inventionis available commercially in toroidal sections. The inventionnevertheless encompasses ducts of different cross-sections.

It has been found that, using the plasma duct of the first aspect of theinvention, it is possible substantially to remove all macroparticlesfrom the plasma beam. Prior art duct designs have incorporated a single45° or 90° bend. These have not totally prevented macroparticles fromreaching the substrate but instead allow significant contamination ofthe deposited layer by macroparticles. Although there is not aline-of-sight from the cathode to the substrate in prior art apparatusthere is nevertheless a single bounce path possible for macroparticleswhich are thus able to reach the substrate after a single bounce on aninner surface of the plasma duct. The plasma duct of the first aspect ofthe invention incorporates two bends and these confer significantlyimproved filtering of macroparticles from the plasma beam.

In order for a macroparticle to reach the substrate via the plasma ductof the invention, at least two successful bounces off the duct wallsmust occur. A large amount of macroparticle kinetic energy is lostduring each bounce and therefore it is found that the probability of amacroparticle entering the duct with sufficient kinetic energysuccessfully to undergo a double bounce on the plasma duct walls and toarrive at the substrate is virtually zero.

This is particularly the case where the two bends are in differentplanes, since the first bend tends to remove particles with momentumsuitable for passing through the second.

Therefore it is a considerable advantage of the invention that improvedfiltering of the beam is achieved and a greater proportion ofmacroparticles are removed from the plasma beam arriving at thesubstrate. With the ability to remove the macroparticles in this way, itis not necessary, for example when depositing diamond-like carbon, toincrease the pressure in the vacuum chamber, which pressure increasewould compromise the properties of the deposited film, and therefore theproperties of the deposited film can be greatly improved.

The first aspect of the invention also provides apparatus for generatinga substantially pure plasma beam including positively charged ions, theapparatus comprising means for generating a plasma beam includingpositive ions from a cathode target and a plasma duct located betweenthe cathode target and a substrate, wherein for a macroparticle enteringthe duct there is no line-of-sight and no single bounce path to the ductexit. The duct has an inner wall and at least two bounces off the wallare needed for a macroparticle to reach the duct exit.

In an embodiment of the invention the duct has a double bend, comprisinga first straight section, a first bend, a second straight section, asecond bend and a third straight section. Such a duct is illustrated inthe examples and, as will be seen, has a serpentine nature. The bendangles in the duct vary independently of each other, provided there isno single bounce path from the entrance of the duct to the exit.

The plasma duct of the apparatus typically has a first bend with anangle of no less than 20 degrees in a first plane and a second bend withan angle of no less than 20 degrees in a second plane. In particularembodiments of the invention the bend angles are, independently, in therange 30-90 degrees, more particularly 35-80 degrees.

Another embodiment of the invention provides apparatus for filtering abeam of positive ions comprising a vacuum chamber, means for generatinga beam of positive ions, a filter duct having an entrance and an exit,and means for steering the beam of positive ions into the entrance ofthe duct and through the duct to the exit, wherein the duct comprises atleast two bends. The two bends of the duct are such that, for a neutralparticle entering the duct, there is no single bounce path to the exit;instead, at least two bounces off inside walls of the duct are neededfor a macroparticle to travel from the duct entrance through the ductand to exit from the duct exit.

In a further embodiment of the invention there is provided apparatus forfiltering a beam of positive ions, comprising means for generatingpositive ions, means for steering positive ions in a beam and a filter,wherein the filter comprises a double bend duct. In a specificembodiment of the invention described in detail below, a cathode arcsource, capable of generating positive carbon ions, is located within avacuum chamber, magnetic means are provided to steer positive ionsthrough the filter and onto a substrate, and the double bend of thefilter duct filters macroparticles from positive ions.

A still further, more particular, embodiment of the invention provides afiltered cathode arc source for depositing positive ions on a substrate,comprising

a vacuum chamber

means in the chamber for generating positive ions from an arc at acathode target

a substrate located on a substrate holder

a filter duct located between the target and the substrate and throughwhich positive ions from the target pass in order to arrive at thesubstrate

means for cooling the cathode

means for applying a positive bias to the filter duct

said filter duct having a first bend and a second bend

means for generating a magnetic field to steer positive ions around thefirst bend, and

means for generating a magnetic field to steer positive ions around thesecond bend.

In a specific embodiment of the invention described in further detailbelow, an arc power supply is connected to the source and provides powerfor both the arc and for arc striking (ignition), achieved using amoveable striker located insider the vacuum chamber. The striker isgrounded, as is the arc anode, and is moved towards the target until atip portion touches or nearly touches the target and ignition occurs.The arc current is around 70-200 amps, and variation in the currentvaries the rate of deposition of ions on the substrate, though therelation is not linear. Arc voltage is material dependent and for anygiven arc source set up does not vary outside a fairly narrow range.With a carbon target the arc voltage is typically about 29V.

It is further an option to provide a plasma duct having a single,graduated radius that does not provide a single bounce path between thetarget and the substrate.

The choice of angles for the double bend plasma duct is to be looked athaving regard to both theoretical and practical aspects. Whilst intheory, any combination of angles can be used, so long as the firstangle is at least 10°, the second at least 10° and the total preferablyat least 50°, in practice a first step is to decide the space angle inlight of the size of the duct. An angle of 10° in a duct 6 inches wideis effectively no angle. A larger duct gives increased transmission buthas to be longer in order to avoid a line-of-sight around each bend.

In use, the inventors have found that a duct diameter of between 4 and 6inches (10 and 15 cm) is particularly suitable. In these ducts, thedouble bends are arranged so that there is no single bounce path for amacroparticle between the target and the substrate.

A duct diameter of 2-3 inches (5-8 cm) provides effective filtering withsmall double bend angles, such as 20 degrees and 30 degrees. On theother hand, a duct of such a small size produces a correspondinglynarrow plasma beam and loses significant amounts of plasma to the ductwall; consequently it has a poor transmission performance compared tolarger diameter ducts. Ducts of these dimensions may therefore beindicated when high filtering efficiency is a priority.

There is also a practical limitation in that the wider the duct thegreater the difficulty in providing a magnetic field to steer theplasma. In use, it has been found that ducts having diameters in the 4inch-6 inch (10 cm-15 cm) range give useable rates of deposition.

In a particular embodiment of the invention, the double bend duct of theapparatus is supplemented by a liner located within the plasma duct. Itis preferable that this liner is removable for cleaning purposes. Inuse, sides of the liner rapidly become coated with a deposit, and, ifthis is allowed to build up over time, particles from the deposittypically fall back into the plasma beam and contaminate the coating onthe substrate. The liner is removable, allowing it to be replaced with aclean liner periodically to prevent this long term deposit from buildingup on the duct walls. The removed liner can be cleaned remote from theapparatus.

In a particularly preferred embodiment of the invention the liner ispositively biased, typically to between 10V-30V. An advantageous effectof this bias is that it creates a repulsion between the liner and thepositive ions in the plasma beam and thereby increases the flow ofplasma through the double bend of the plasma duct. Thus, any reductionin plasma beam transmission due to the double bend of the apparatus canbe at least partially compensated by biasing the duct liner.

In a specific embodiment of the invention, the double bend plasma ductis lined with a liner adapted further to increase filtering ofmacroparticles from the plasma beam. The liner is made-up of a series ofrings having flanges that project outwards into the interior of the ductand are angled backwards and towards the target. The liner is made of aseries of rings linked, alternately around their peripheries, forexample at 12 o'clock and 6 o'clock, and then at 3 o'clock and 9o'clock. This liner is flexible and suitable to be pushed into the ductand around the duct bends.

In another particular embodiment of the invention, the apparatus furthercomprises baffles located at or close to walls of the vacuum chamber ofthe apparatus and between the cathode target and the plasma duct. It isobserved that most macroparticles are emitted from the cathode target ata large angle and these are prevented from hitting the target walls, orentering the plasma duct at a large angle, by the baffles.Macroparticles and plasma emitted substantially normal to the cathodetarget surface pass through the baffles.

It is preferred that the baffles are non-conducting, e.g. made of PTFE;initial arrival of positive ions establishes a positive charge thatexerts an electrostatic repulsive effect against the positive ions inthe plasma and increases the flow of plasma including positive ions intothe plasma duct. It is particularly preferred that the baffles areremovable for cleaning purposes. The baffles quickly become coated inuse with a fine, particulate coating and if this is allowed to build upover a long time it can contaminate the plasma beam. Baffles that areremovable can be immediately replaced with clean baffles and the dirtyones can be cleaned remote from the apparatus.

In use of the filtered cathode arc source of the invention, it isdesirable to provide as short as possible a mean free path for carbonions in the plasma beam. This can be achieved by providing a vacuumpressure that is suitably high, typically of the order of 10⁻⁶ Torr,though during deposition this pressure may typically be around 10⁻⁵ Torror even slightly less if gas is let into the vacuum chamber. In typicaloperation, the vacuum chamber is pumped down to about 2×10⁻⁶ Torr andduring deposition the pressure may rise slightly towards 10⁻⁵ Torr. Themean free path of particles in the plasma beam can further be reduced byproviding and designing apparatus having as short as possible a distancebetween the cathode target and the substrate.

The plasma beam is guided through the plasma duct by methods andapparatus known in the art. For example, the beam can be guided by acurvi-linear magnetic field along the length of the duct. Alternatively,the plasma beam is guided by a crossed electric and magnetic field.Coils, when provided on the plasma duct double bends, to provide themagnetic steering field for the plasma beam, are optionally watercooled. Permanent magnets are another option.

Using the apparatus of the invention, it is possible to achieve adeposition rate of up to about 15 angstroms per second over an area of25 in² (157 cm²) using an arc current of about 120 A and having afloating duct bias. The magnetic field from the coil current, i.e. thefield inside the double bends, is about 60 mT on the straight piecesbetween the bends and about 40 mT on each of the two bends.

The macroparticle count of TAC films produced using the method andapparatus of the invention have been measured and found to be less than100 per cm² in a coating 600 angstroms thick. These values comparefavourably with those in U.S. Pat. No. 5,401,543, in which examples 1-3disclose particle counts of about 15,000 per cm² for a coating ofthickness 500 angstroms. This latter figure is a typical figure for thelevel of macroparticles in films of the prior art.

A second aspect of the invention relates to the application of a coatingof positive ions onto an optical element. According to the second aspectof the invention there is provided apparatus for applying a coating ofpositive ions to a dielectric substrate, the apparatus comprising:

means for generating an arc at a cathode target, the cathode targetcontaining the ions to be deposited on the dielectric substrate,

magnetic means for directing a beam of ions emitted from the cathodealong a filter path substantially to remove macroparticles therefrom,

means for holding the dielectric substrate in the filtered ion beam,

means for applying an RF bias to the dielectric substrate to dissipateelectrostatic charge accruing on the dielectric substrate by depositionof positive ions.

Whereas prior art coatings were not considered of sufficiently highquality to obtain optical and electronic coatings, the filtered plasmabeam of the invention is of such purity that high quality coatingssuitable for optical use can be obtained.

Examples of these coatings include tetrahedral amorphous carbon (TAC)and silicon coatings, and oxides, nitrides, selenides and carbides, suchas silicon dioxide, aluminium oxide, titanium oxide, titanium nitride.

In an embodiment of the second aspect of the invention, there isprovided apparatus for depositing a multi-layer coating of positive ionson a dielectric optical element, the apparatus comprising a firstcathode target and further comprising a second cathode target and meansfor interchanging the first and second cathode targets without breakingvacuum in the apparatus. In use a first coating of ions from the firsttarget is applied, targets are changed without breaking vacuum and thena second coating of ions is applied from the second target. The filterpath optionally includes a double bend toroidal duct adaptedsubstantially to remove macroparticles from the plasma beam.

The duct conveniently incorporates the first aspect of the invention. Ina particular embodiment of the second aspect of the invention, the ductis lined with a liner, preferably a non-magnetic liner. More preferablythe liner is removable from the duct for cleaning. In anotherembodiment, the liner is given a positive bias, preferably within10V-30V.

The invention enables deposition of a film of TAC that is substantially,and can potentially be entirely, free of macroparticles. This film canbe obtained without the need to introduce into the vacuum chamber a gasto prevent formation of macroparticles, which gas routinely leads to adeterioration in the properties of the deposited TAC. Also, in otherapplications operation using reduced gas volume is made possible. TheTAC coatings of the invention can be deposited with the followingcharacteristics:

(a) smoothness root mean square of 3 angstroms average deviation fromsurface, for a TAC coating deposited on a polished silicon surface.

(b) hardness 65 GPa (this compares to a value of 100 for pure diamond).Typical hardness for the TAC coatings is in the range of 55-75 GPa, fora film on silicon. The sp³ content of films of the invention istypically at least 80%, and in embodiments of the invention is typicallyat least 85%, and in some specific embodiments as high as 88%. Theincreased sp³ content of films of the invention compared to prior artfilms gives increased hardness. For example, the films produced by theequipment of Falabella et al (U.S. Pat. No. 5,279,723) are reported tohave sp³ content of around 60%.

(c) refractive index 2.5-2.6 (compared to the value for diamond of 2.4).

(d) optical band gap approximately 2 eV.

(e) transmittance 70% in the infra red spectrum.

(f) Young's modulus 700 GPA, typically 600-800 (compared to 1010 GPA fordiamond).

(g) friction coefficient of sapphire of TAC film of approximately 0.15.

(h) density 3 g/cubic cm. The density of pure, natural diamond isnormally 3.5 g/cubic cm. The density of TAC films of the invention istypically at or above 3 g/cm³ and preferably 3.2-3.3 g/cm³ or above.Prior art publications have reported diamond like carbon films having adensity of around 2.7 g/cm³.

It is a particular advantage that the TAC films of the invention possesthese characteristics and that the films do not compromise any of thesecharacteristics as a result of efforts to remove macroparticles from thefilms. Macroparticles are substantially entirely removed by the improvedfiltering of the invention, so all these characteristics can be obtainedin the deposited film. It is particularly noticeable that the TAC filmsof the invention do not absorb at wavelengths associated with C--H IRabsorption, indicating no hydrogen in the TAC films.

A further aspect of the invention accordingly provides a TAC filmpossessing the above-identified film characteristics. The film depth istypically 50-1000 angstroms.

In a third aspect, the invention relates to obtaining multi-layercoatings on a substrate. By multi-layer coating it is intended toindicate that the substrate has a first coating of one material, such assilicon dioxide and a second coating of a different material, such asaluminium oxide. Multi-layer coatings are of particular use in theoptical and electrical field in which the combination of propertiesobtained by the individual coatings produces an overall property of thecoating which is of a particular application.

A third aspect of the invention provides a cathodic arc source forgenerating a plasma beam from a cathode target, comprising:

a vacuum chamber,

a first cathode located at a cathode station within the chamber,

means for generating an arc from the cathode at the cathode station,

a second cathode located within the chamber, and

means for interchanging the first and second cathodes without breakingvacuum.

Known cathodic arc sources incorporate a single cathode target. Toproduce multi-layer coatings it is normally necessary to break vacuum inthe chamber, replace the cathode target, pump down to re-establishvacuum in the chamber and then apply the second coating layer. As willbe appreciated, the third aspect of the invention enables multi-layercoatings to be obtained without the expensive and time consuming step ofbreaking and then re-establishing vacuum in the chamber.

It is known to provide a first coating layer of a metal in combinationwith one gas and then a second coating layer of the same metal and asecond gas. For example, a silicon nitride coating may be applied on topof a silicon dioxide coating. Other such combinations will be known tothose skilled in this art. To obtain such a multi-layer coating thecathode target is made of the same material. However, once a cathodetarget has been used with a first gas then it becomes contaminated withtraces of that first gas. When that same contaminated cathode is usedwith a second gas, the initial plasma beam from the cathode iscontaminated with ions of the first gas. In this circumstance, the thirdaspect of the invention is also of advantageous use because the firstcathode can be exchanged with a second cathode of the same material, butthe second cathode is not contaminated with the first gas. Again, thisexchange is achieved rapidly and efficiently without breaking vacuum inthe chamber.

The second cathode can be one of a plurality of cathodes, the sourcecomprising means for interchanging the first cathode with any of theplurality of cathodes without breaking the vacuum. Optionally, thesecond cathode is located in a cathode magazine.

In a particular embodiment of the third aspect of the invention there isa vacuum lock between the cathode magazine and the cathode station.

The vacuum chamber advantageously may further comprise first and secondvacuum compartments; the cathode magazine is located within the secondvacuum compartment; the cathode station is located within the firstvacuum compartment; and the source comprises means for separateevacuation of the first and second vacuum compartments. It is preferredthat the second compartment comprises a port allowing access to thecathode magazine without breaking vacuum in the first compartment.

In use of this particular embodiment, an operator is able to gain accessto cathodes in the cathode magazine without breaking vacuum in thechamber holding the cathode from which an arc is to be generated. Thisenables convenient exchange of cathodes into the magazine without thetime-consuming need to break and re-establish vacuum in the main chamberin the apparatus. The time taken to replace an exhausted cathode in acontinuous coating process is reduced and multi-layer coating ofsubstrates is greatly facilitated.

In a particular embodiment of the invention to be described further indetail below, the cathode interchanging means comprises a cathodegripping device mounted on a target transport arm and adapted totransfer a cathode between the cathode station and the cathode magazine.

A fourth aspect of the invention relates to coatings such as silicondioxide or aluminium oxide that have metallic and gas components.

According to a fourth aspect of the invention apparatus for applying acoating of positive ions to a substrate comprises:

means for generating an arc at a cathode target, the cathode containingone component of the coating,

means for introducing into the arc a gas component of the coating, and

magnetic means for directing a beam of ions via a filter path to thesubstrate, the filter path adapted substantially to extractmacroparticles from the ion beam.

This apparatus enables gas ionisation to occur in the arc. Conveniently,the gas introducing means comprises a gas inlet proximal to the cathode.

Ionisation in the plasma ball of the cathode arc is fierce. Thus, gasintroduced into the arc is rapidly and efficiently ionised andcontributes positively charged gas ions to the plasma beam. By priorcalibration of the rate of gas input a plasma beam is obtainedcontaining metallic positive ions and gas positive ions in a suitablestoichiometric ratio to produce a composite metal-gas coating on thesubstrate. The fourth aspect of the invention enables highly efficientuse of gas and can be adapted to ensure that gas use is at the mostefficient level to obtain the required composite gas-metal coating onthe substrate.

Apparatus according to the fourth aspect further removes the requirementfor there to be a gas inlet close to the substrate or as part of theplasma duct, thereby simplifying construction of the substrate holderand of the plasma duct.

Examples of suitable gases for obtaining gas-metal composite coatingsinclude oxygen, nitrogen, hydrogen and methane.

Introduction of gas into the arc of the cathodic arc source canadvantageously be combined with other aspects of this inventiondescribed above. In a particular embodiment of the fourth aspect of theinvention, the cathode is a first cathode located at a cathode station,the station being positioned so that an arc can be generated from acathode located at the station, the apparatus further comprising asecond cathode located in a cathode magazine, the apparatus furthercomprising means for interchanging the first cathode with the secondcathode without breaking vacuum in the vacuum chamber.

Apparatus according to the fourth aspect can be adapted for applyingmulti-layer coatings to the substrate, one layer of the coatingcomprising ions from the first cathode and another layer of the coatingcomprising ions from the second cathode, the multi-layer coatingachieved without breaking vacuum in the vacuum chamber.

A fifth aspect of the invention relates to arc ignition avoidingdrawbacks of previously available igniters.

A fifth aspect of the invention provides apparatus for applying acoating of positive ions to a substrate, the apparatus comprising:

means for generating an arc at a cathode target,

means for directing a beams of ions emitted from the cathode to thesubstrate, via a filter path substantially to remove macroparticlestherefrom, and

arc ignition means comprising an ignition anode retracted towards a wallof the apparatus and movable between this retracted position and anignition position in contact with the cathode surface,

and means for storing the position at which arc ignition occurred.

It is preferred that movement of the arc ignition means is by a steppermotor, giving accurate control.

An embodiment of the invention described below includes arc ignitionmeans that comprises means for moving the ignition anode towards theignition position, means for determining arc ignition and means forreturning the arc ignition means to its retracted position after the archas ignited. It is an option for the ignition anode to be on the end ofan igniter arm, the other end of which is rotatably mounted on theapparatus.

In use, if a first arc ignition position is known, in subsequent arcignition the ignition anode is moved directly to this position andthereafter moved towards an ignition position of the cathode.

On first ignition of an arc from a cathode target, the arc ignitionmeans is typically moved towards the cathode at a slow rate, theoperator attempting to avoid contact with the cathode target that wouldimpact upon the target and risk damaging the target. Once an arc isignited from a cathode then the position of arc ignition can be noted.Subsequent ignition of the same cathode is achieved conveniently bymoving the anode on the arc ignition means towards the known firstignition position and thereafter slowly moving the anode towards aposition in which subsequent cathode ignition occurs. As a cathode isconsumed during use it is usual for subsequent ignition of the cathodeto occur at a lower position than the position of first cathodeignition.

In an embodiment of the invention a controlling circuit monitors boththe arc voltage and the movement of the arc ignition means. Thecontrolling circuit advances the arc ignition anode towards the cathodeand when arc ignition is sensed by change in the arc voltage thecontrolling circuit retracts the arc ignition anode away from thecathode.

Thus, the arc ignition means of the fifth aspect of the inventionprovides a convenient way to re-ignite an arc from a partially consumedcathode. The problems of prior art ignition devices that often led tocontamination of the cathode or required an observer to look through asubstantially opaque viewing window are overcome.

It has been observed that ignition of the arc using the igniter of theinvention can be achieved at a vacuum pressure of 10⁻⁴ torr. As is knownin the art, this vacuum level is suitable for some coatings but forobtaining coatings of exceptional hardness and uniformity a highervacuum is routinely required.

Apparatus of the invention enables the production of coatings known inthe art with improved coating properties due to the improved puritythereof.

As will be appreciated, preferred particular embodiments of theinvention combine the various aspects of the invention in all possiblecombinations. A very particular, specific embodiment of the inventioncombines all of the aspects of the invention in a single filteredcathodic arc source.

In another embodiment of the invention, the cathode target is arrangedsubstantially horizontally, with the plasma beam of positive ionsemitted substantially vertically therefrom. The cathode target isretained by a guide. This arrangement allows the cathode target to meltduring preparation of the cathodic arc and therefore this embodiment ofthe invention is suitable for use with low-melting point targets such ascopper and aluminium.

Prior art cathodic arc sources were not suitable for use withlow-melting point targets for various reasons. Firstly, the layout ofthe cathodic arc source forced the target to be held vertically or at anangle to the horizontal, so that if a low-melting point target were usedit would melt and flow away from the cathode station and drip on to thewalls or the bottom of the source. Secondly, production ofmacroparticles from a molten cathode target is higher than from a solidcathode target. The prior art devices did not provide an efficientenough filter of macroparticles to enable acceptable quality coatings oflow melting point metals to be obtained from a molten cathode.

In some isolated instances, prior art devices have produced coatings ofaluminium, however these have been produced using a pulsed cathodic arcsource, as continuous use of the arc would result in melting of thecathode target. Using a water-cooled anode according to a furtherembodiment of the invention, and using the cathodic arc source forperiods of up to 2, even 5 minutes, coatings of low-melting pointcathodes can be obtained using the apparatus of the invention. Thus, awhole new field of coating technology has been opened up by thisinvention.

In a particularly preferred embodiment of the invention, the filteredcathodic arc generates pure plasmas to deposit dense and clean thinfilms. The plasma is emitted from cathodic arc spots on the surface of aconsumable cathode and guided by a radial electric field and acurvilinear axial magnetic field through a positively biased double bendtoroidal duct to a substrate in a coating vacuum chamber. Unwantedmacroparticles emitted with the plasma from the cathode are effectivelyfiltered out by a set of removable ceramic baffles, a removablestainless steel bellows and a double bend toroidal duct. The cleanplasma beam virtually without macro-particles at the exit of the ductcan be scanned in one dimension by a beam scanning system. This scannedbeam, in combination with a rotating substrate holder in the coatingvacuum chamber, can deposit a large area of films with good uniformityat room or any other desired substrate temperatures. Ions of thedeposited material with a desired energy can be extracted from theplasma beam by using a DC or RF bias on various types of substrates,such as metals, semiconductors, plastics, ceramics and glasses.

The plasma beam from the filtered cathodic arc source of the inventionis suitable for room temperature coating of substrates, thus enablingcoating of an almost unlimited range of substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

There now follows a brief description of specific embodiments of theinvention, illustrated by drawings in which:

FIG. 1 shows a schematic side view of a filtered cathodic arc assemblyof the invention;

FIG. 2 shows a schematic top view of a Plasma Beam Scanning System and adouble bend duct of a filtered cathodic arc of FIG. 1.

FIG. 3 shows a schematic side view of a multi-target changer with theanode and cathode assembly of the filtered cathodic arc.

FIG. 4 shows the infra red absorption spectrum of a TAC film of theinvention: wave number on the X axis and absorbtion co-efficient on theY axis, illustrating C--C absorption at about 1210-837 and no C--Habsorbtion.

FIG. 5 shows the morphology of a TAC film of the invention under atomicforce microscope.

FIG. 6 shows the Raman spectrum of a TAC film according to theinvention: the amorphous diamond structure of films of the invention isevidenced by the combined peak at 1400-1600. The X axis shows Ramanshift (cm⁻¹) and the Y axis shows intensity (cPS).

FIGS. 7-10 show calculated transmission and filtering efficiencies ofdouble bend ducts of the invention (x axis--first bend angle; yaxis--second bend angle; FIGS. 7 and 9--transmission; FIGS. 8 and10--filtering);

FIG. 11 shows a duct liner of the invention;

FIGS. 12-15 show double bend ducts of the invention.

FIGS. 16 and 17 show deposition apparatus incorporating FCAs accordingto the invention; and

FIGS. 18-23 are photographs comparing coatings produced using apparatusof the invention (FIGS. 18, 19) with coatings of the prior art (FIGS.20, 21) and coatings made with a single bend filter duct (FIGS. 22, 23).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

1) Cathode Assembly

Referring to FIG. 1, the cathode assembly is made up of a water cavityblock (1) with a mounting flange (2). A consumable cathode target (3) islocated on the top of the water cavity block (1). The cathode target issurrounded by a ceramic guard ring (4) that prevents the arc spots onthe target surface from migrating off the target surface (3).

The cooling water goes in from the water inlet (8) and returns throughoutlet (9) to cool the whole cathode assembly. The water inlet andoutlet are mounted on a base flange (10) that is electrically insulatedfrom the cathode.

The cathode assembly is mounted onto the anode assembly by theelectrically insulated mounting flange (2). The cathode assembly iselectrically connected to the negative terminal of an arc power supply(11) and is vertically mounted so that the low melting point materialscan also be used as cathode targets (3).

2) Anode Assembly

The anode assembly is made up of a straight piece of duct with a waterjacket (12) known in the art for cooling. Located between andelectrically insulated from the cathode assembly and the double bendfiltering system, it is electrically grounded and connected to thepositive terminal of an arc power supply (11).

A view port (13) is placed in the vicinity of the target (3). Anorthogonal placed gas inlet port (14) is located beside the view port(13). The gas in the target area (3) undergoes maximum ionization by theplasma spot. The reactive or doping gas can be input from the gas inletport (14) to achieve maximum ionization, hence highest reaction rate ordoping efficiency.

A set of removable ceramic baffles (15) are located in the upper part ofthe anode assembly. They are used for three reasons: firstly, to preventsome of the macroparticles from rebounding off the wall and entering thefiltering system, secondly, to reduce the loss of positive ions to thewall, thus to increase the plasma throughput, thirdly, the baffles areremovable so that built up materials can be easily cleaned off.

The magnetic coil (16) around the anode creates an axial magnetic fieldthat focuses the plasma beam and guides it into the next stage, thefiltering system.

3) Automatic Arc Triggering System

The automatic arc triggering system is located on top of the anodeassembly seen as a bulge. It is made up of a striker (17), a triggeringtip (18), a stepper motor (not shown in the figure), and a controller(19). There is a narrow opening on the water jacket wall of the anode(12) to allow the striker (17) to descend to the target (3).

At the start of the triggering, initiated by the controller (19), thestriker (17) rotates towards the surface of the target (3) while thecontroller (19) continuously monitors the change of the status in arcpower supply (11). When the triggering tip (18) touches the targetsurface (3) and subsequently the arc is ignited, the controller (19)senses the change in arc power supply (11) and lifts the striker (17)away from the target surface (3) and away from the plasma beam to avoidcontaminating the plasma. The position of the triggering tip (18) at arcignition is stored by the controller. When re-ignition is required thecontroller is able to drive the triggering tip (18) rapidly to thestored position without risk of fracturing the cathode target. Anyfurther movement necessitated by evaporation of the cathode target, canbe at a much slower rate.

4) Filtering System

Referring to FIG. 2, the filtering system comprises a double bendtoroidal duct (20) and a removable stainless steel bellows (21) (shownin FIG. 1) fitting well inside the wall of the duct, which is mountedbetween and electrically insulated from the anode assembly and plasmabeam scanning system.

The bias power supply (22) gives positive bias to the duct (20) andproduces the radial electric field within the plasma beam across theaxial axis. It is seen from computer simulations that an appropriateradial electric field is very important in efficient transportation of aplasma beam. The magnetic coil (23) generates a curvilinear axialmagnetic field along the duct. This crossed electric-magnetic fieldefficiently guides the plasma through the double bend and effectivelyfilters out unwanted macroparticles coming into the filtering system andneutral atoms. A removable duct liner (21) can enhance the filteringefficiency. It is again removable to allow for easy maintenance of theduct. Eventually, only a pure plasma beam with known ion energy willexit from the filtering system. It is this pure plasma beam that canproduce a large variety of thin films with high quality and/orunconventional properties.

5) Plasma Beam Scanning System

Referring to FIGS. 1 and 2, a magnetic plasma beam scanner is locatedafter the filtering system. The scanner is made up of a C-shaped softmagnetic core (24) with two open poles on the top and bottom of thestraight duct (25). There is a solenoid around the magnetic core with anelectronic driving unit (26) to generate a magnetic field between thetwo poles. This magnetic field deflects the plasma beam. A varyingmagnetic field sweeps the plasma beam up and down. The followingcone-shaped or fan shaped duct (27) is to allow the beam to be scanned.This fast up and down sweeping plasma beam can deposit large areas offilms with good uniformity. It is also possible, with an appropriatedrive signal to the scanner, to compensate for variations in ion densityacross the width of the filter output. The plasma beam can be eitherscanned horizontally or vertically. It can also be scanned bothhorizontally and vertically if two magnetic scanners are installed. Withthe plasma beam scanning system there are few limitations to the size ofcoatings as far as this attachment is concerned. The main limitationarises from the size of the coating chamber.

The ions of the deposited material in the plasma beam can be depositedon various types of substrates, such as metals, semiconductors,plastics, ceramics and glasses, etc. The substrate can be further biasedby using a DC or RF bias to extract ions with a desired energy from theplasma beam. The deposition can be carried out at room or any otherdesired substrate temperature.

6) Multi-Target Changer

Referring to FIG. 3, the multi-target changer enables easy and efficientdeposition of multi-layer coatings. It is basically a load-lock systemcomprising of a vacuum chamber (28) with access door to the targetswhich are stored and indexed in the storage rack (29). A lineartransport arm (30), located inside the duct (31) connecting the storagechamber to the anode assembly via a gate valve (not shown in thefigure), is used to transfer the targets. A gripper (32) is designed tohold and support the target during the transfer.

EXAMPLE 2

The electrostatic potential difference between the duct wall and theplasma centre was added to the drift model equations developed bySchmidt and Khizhnyak (Phys. Fluids, 3, 961 (1960) and SovietPhys.--Tech. Phys., 10 (5), 655 (1965)). ##EQU1##

The first terms on the right hand side of these two equations describethe motion of electrons and ions in the electromagnetic field. Thesecond terms describe the accumulative effect of interactions betweenelectrons and ions. The last terms describe the effect of the radialelectric field between the duct wall and the centre of the plasma.

These two equations are used to simulate the motion of the plasma in thetoroidal duct and used in design of the filtered cathode arc systemaccording to the present invention. The motion of neutral particles inthe duct was simulated on a computer according to a specular reflectionwhen a neutral particle hits the duct wall.

The results of these two simulations, namely (1) the effect ofvariations in the angles of the first and second bends upon plasmatransmission through the duct and (2) the effect of variation in thesetwo bend angles upon filtering of macroparticles from the plasma, areillustrated in FIGS. 7, 8, 9, and 10.

According to the model, a double bend plasma duct of the invention canoutperform a single bend in terms of plasma transmission (FIGS. 7, 9)and/or macroparticle filtering (FIGS. 8, 10).

FIGS. 7 and 8 are calculations for a duct in which the angle between thedirection of plasma entering and exiting the duct is 90°, that is to saythe net effect of two bends in the duct. In FIGS. 9 and 10 the angle is45°.

EXAMPLE 3

Referring to FIG. 11, a duct liner (shown generally as 51) has rings(52) and linkers (53). Each ring is circular in cross-section and has aninner lip (54) extending inwardly and downwardly. In use the liner isarranged within a plasma duct such that this lip is directed towards thetarget, ie against the plasma flow. The linkage between adjacent ringsis by linkers (53) connected to each ring. These linkers are arranged indifferent places around the peripheries of adjacent rings to give theliner enough flexibility for it to be pushed into a toroidal duct andaround the bends of the duct. The liner is made of stainless steel.

EXAMPLE 4

Referring to FIGS. 12-15, double bend ducts are shown generally as 200.

FIGS. 12-14 show a duct in which the angle of plasma entering the ductis at 90 degrees to plasma exiting the duct. FIG. 12 is a view fromabove, FIG. 13 a perspective view and FIG. 14 a view from the side, allof the same duct. In FIG. 15 that angle is 45 degrees.

The duct in FIGS. 12-14 has straight sections (202, 204 and 206),connected by toroidal bends (208 and 210). The angle of bend 208 isabout 50 degrees and of bend 210 is about 60 degrees. There is an angleof 90 degrees between the plane of the first bend and the plane of thesecond.

The duct in FIG. 15 has straight sections (220, 222 and 224) connectedby toroidal bends (226 and 228). Bend 226 has an angle of about 35degrees and bend 228 an angle of about 40 degrees. There is an angle of45 degrees between the duct entrance plane and the duct exit plane.

EXAMPLE 5

Referring to FIGS. 16 and 17, deposition apparatus (100) is illustratedcomprising filtered cathode arc sources having double bend filter ducts.The apparatus has a vacuum chamber (120) and two FCA sources (102, 111).The first source (102) has a cathode and anode (101) for generating anarc from a target (not shown). Positive ions from the target arefiltered by a double bend duct comprising a first straight section(103), a first bend (104), a second straight section (105), a secondbend (106) and a third straight section (107) that opens into the vacuumcoating chamber (120). Both ducts are toroidal in cross-section and havea double bend preventing a line-of-sight from the target to thesubstrate and preventing also a single bounce path from the target tothe substrate. Ports (121) on the chamber allow visual inspection ofsubstrates mounted on a rotatable drum (not shown) inside. Positive ionsare steered through the duct by a magnetic field produced from coilwindings around the whole length of the duct.

The first bend (104) has an angle of 50 degrees and the second bend(106) has an angle of 60 degrees. These two bends are in differentplanes, such that the resultant angle between (i) plasma entering theduct and passing through the first straight section (103) and (ii)plasma passing through the third straight section and exiting the ductis 90 degrees.

Likewise, positive ions from the second source are filtered by a doublebend duct having first, second and third sections (112, 114, 116) andfirst and second bends (113, 115). In the case of the second source, thefirst bend (113) has an angle of 35 degrees, the second bend (115) hasan angle of 40 degrees and the resultant angle between the first andthird straight sections (112, 116) is 45 degrees.

A frame (122) bears the coating chamber and the two FCA sources.

EXAMPLE 6

To test the macroparticle filtering ability of the invention, samplecoatings were made using the double bend filter duct of the inventionand compared with coatings from prior art apparatus and with coatingsmade using apparatus having just one 90 degree filter duct bend. Theresults are shown in FIGS. 18-23.

All of FIGS. 18-23 are photographs taken at a magnification of ×125 ofdeposited layers of TAC that are 600 Angstroms thick.

Both FIGS. 18 and 19 are layers deposited using a double bend ducthaving in and out pieces at 90 degrees, a first bend angle of about 50degrees and a second bend angle of about 60 degrees. No macroparticles,which would be visible as dark or black blemishes on the photograph, areseen.

Both FIGS. 20 and 21 are films made using filtered cathode arc equipmentcommercially available from Commonwealth Scientific Corporation.Numerous contaminating macroparticles, visible as black splodges, areevident.

Lastly, both FIGS. 22 and 23 are films made using filtered cathodesource apparatus built by the inventors that has a single 90 degree bendin the filter duct. Again, numerous contaminating macroparticles areevident in the film.

Variations and modifications from the described specific embodimentswill be apparent from the description to a person of skill in the artand consequently the invention is not to be construed as limited to anyspecific embodiment.

We claim:
 1. Apparatus for filtering macroparticles from a beam ofpositive ions, comprising means for generating a beam of positive ionsfrom a plasma source, a filter duct and means for steering positive ionsthrough the duct, said duct having two bends.
 2. The apparatus of claim1, wherein the first bend is at least 20 degrees.
 3. The apparatus ofclaim 1, wherein the first bend and the second bend are at least 20degrees.
 4. The apparatus of claim 3, further comprising a removableliner within the duct and biased at a positive voltage.
 5. The apparatusof claim 3, further comprising a vacuum chamber, means in the chamberfor generating an arc at a cathode target for generating positive ions,a substrate distal from the target and means for steering positive ionsfrom the target through the duct and onto the substrate.
 6. Theapparatus of claim 5, wherein the target is graphite and the apparatusis for depositing a layer of tetrahedral amorphous carbon on thesubstrate.
 7. The apparatus of claim 5, comprising magnetic means forsteering positive ions through the plasma duct.
 8. The apparatus ofclaim 1, wherein the duct comprises a first bend in a first plane and asecond bend in a second plane, and wherein the first and second planesare not co-incident.
 9. The apparatus of claim 8, wherein there is anangle of at least 20 degrees between the first and second planes. 10.The apparatus of claim 1, for applying a coating of positive ions to adielectric substrate, the apparatus further comprising:means for holdingthe dielectric substrate in the filtered ion beam, and means forapplying an RF bias to the dielectric substrate to dissipateelectrostatic charge accruing on the dielectric substrate by depositionof positive ions.
 11. The apparatus of claim 10, for coating adielectric optical substrate.
 12. The apparatus of claim 10, fordepositing a multi-layer coating of positive ions on a dielectricoptical element, the apparatus further comprising a second cathode andmeans for interchanging the first and second cathodes without breakingvacuum in the apparatus.
 13. The apparatus of claim 10, comprising afilter path that includes a double bend toroidal duct adaptedsubstantially to remove macroparticles from the plasma beam.
 14. Theapparatus of claim 13, wherein the duct is lined with a non-magnetic,removable liner.
 15. The apparatus of claim 14, wherein the liner biasis modulated to within 10V-30V.
 16. Apparatus for filteringmacroparticles from a beam of positive ions, comprising means forgenerating a beam of positive ions from a plasma source, a filter ducthaving an entrance and an exit and means for steering positive ionsthrough the duct, wherein there is no line-of-sight and no single bouncepath from the entrance to the exit of the duct.
 17. A method ofgenerating a substantially pure plasma beam including positively chargedions, the method comprising the steps of:generating a plasma beamincluding positive ions from a cathode target, and passing the beamtowards a substrate through a plasma duct having a first bend with anangle of no less than 20° in a first plane and a second bend with anangle of no less than 20° in a second plane.
 18. The method of claim 17,wherein the first and second planes are not co-incident.
 19. The methodof claim 18, wherein the first and second planes are at an angle ofbetween 30-90° to each other.
 20. The method of claim 18, wherein thefirst bend has an angle of no less than 40 degrees.
 21. The method ofclaim 18, wherein the second bend has an angle of no less than 40degrees.
 22. The method of claim 18, wherein the duct further comprisesa removable liner.
 23. The method of claim 22, wherein the liner is ofstainless steel.
 24. The method of claim 22, wherein the liner is biasedat 10V-30V.